Switchable and Tunable Ferroelectric Bulk Acoustic Wave Resonators and Filters

Ferroelectric materials such as barium titanate (BaTiO 3 or BTO), strontium titanate (SrTiO3 or STO), and their solid solution barium strontium titanate (BaxSr1-xTiO 3 or BST) have been under investigation for over 50 years. BTO, STO, and BST are high-k dielectric materials, with a field dependent permittivity and a perovskite crystal structure. At room temperature BTO is a ferroelectric with a ferroelectric to paraelectric transition temperature of about 116°C (Curie temperature), while STO has no ferroelectric phase. The formation of a solid solution between BTO and STO allows for the engineering of the Curie temperature; the Curie temperature decreses as the mole ratio of barium decreases. Extensive research went into understanding the properties of BST and developing RF circuits such as tunable capacitors, tunable matching networks, tunable filters, phase shifters and harmonic generators.;BST tunable capacitors have always had anomalous resonances in the one port scattering parameter measurements, although they are very small they degrade the quality factor of the device, and research went into reducing these resonances as much as possible.;The goal of this thesis is to investigate these anomalous resonances and exploit them into RF devices and circuits. Careful investigation showed that these resonances were field induced piezoelectric resonance. Piezoelectric materials such as AlN, ZnO, and PZT are used in many applications, such as resonators, and filters. Thin film bulk acoustic wave resonators (FBAR) have been in use by research and industry since the early 1980s, and in high volume production for cell phone duplexers since early 2000s. FBAR filters and duplexers have several advantages over surface acoustic wave (SAW) and ceramic devices such as high quality factors necessary for sharp filter skirts, small size, high performance, and ease of integration. There are two approaches to designing bulk acoustic wave resonators. The first is an FBAR where a piezoelectric material is sandwiched between two metal electrodes and an air interface at the electrodes. The second approach is the solidly mounted resonator (SMR) where the piezoelectric material is deposited between two metal electrodes and an air interface at the top electrode and an acoustical Bragg reflector or acoustic mirror interface at the bottom electrode. The SMR approach was chosen because of its mechanical and high power durability and best choice for integration with monolithic devices.;Modeling of bulk acoustic wave resonances using the Mason model and the Butterworth-Van Dyke model will be discussed. A process has been developed to fabricate BST and STO voltage activated bulk acoustic wave resonators and filters. A C-band voltage activated bulk acoustic wave filter will be demonstrated with insertion loss of -4.26 dB and return loss of -13.5 dB.